All the peripherals receive one or several clocks that are generated via RCC internal peripheral. RCC relies on several clocks sources (LSI, LSE, HSI, HSE, CSI) and four PLL in order to provide adequate input frequencies to all the peripherals. The clock tree covers all the system clock distribution aspects, from the clock sources to the consumer peripherals (internal and external), except clock gating management that is locally controlled by each peripheral driver.
1. Overview
The clock tree is managed via RCC internal peripheral hardware block and it is configured at different steps from the Cortex-A7:
- When the device is reset, all RCC registers take their reset values: the four PLL are disabled and most of the clock source selectors are pointing to the HSI.
- The ROM Code configures the minimum clock tree needed to boot on the selected boot device.
- TF-A BL2 has the same strategy as the ROM code, configuring the minimum clock tree needed for its execution, thanks to the configuration given in the device tree.
- OP-TEE completely configures the clock tree as expected, all the way up to Linux, thanks to the configuration given in the device tree.
- Linux may partly modify clock tree at runtime thanks to SCMI clock services provided by OP-TEE secure OS. Information related to SCMI are taken from the device tree:
- Linux binding is available in Documentation/devicetree/bindings/clock/clock-bindings.txt (and surrounding files): 'fixed-clock' compatible, 'clocks' and 'assigned-clocks' properties are important concepts to understand the management of clocks providers/consumers.
2. How to build a clock tree?
Building a clock tree is quite complex as it needs to take into account the constraints set by each internal and external peripheral, including external clock sources.
We encourage the use of STM32CubeMX to build the clock tree, and avoid having to know all internal peripherals details: the tool allows to select the peripherals that will be present on the board, fix the clock sources frequencies and automatically find an optimized clock tree. It is then able to generate the device tree that is directly consumed by the boot chain and the secure OS. Linux kernel will be able to modify clock tree at run time thanks to SCMI clock services provided by OP-TEE.
3. ST boards clock tree
This chapter ensures that all peripherals receive clocks with characteristics compatible with the specification (frequency, duty cycle, precision) on each ST board.
 Information
|
| See How to change the CPU frequency article for more information about the CPU frequency setting (PLL1P), including dynamic voltage and frequency scaling (DVFS) configuration. |
3.1. STM32MP135F-DK Discovery kit 
case
This chapter shows the result of the boot time clock tree set by the FSBL, overlaid by the run time clock tree set by the Secure OS on STM32MP135F-DK Discovery kit .
Linux eventual runtime modifications are not covered here.
3.1.1. Clock tree
The following table shows what STM32MP135F-DK Discovery kit clock tree looks like, as a result of the boot chain execution with the device tree built with STM32CubeMX.
Component | Parent | Frequency | Used? | Comment |
-----------------------|---------|-----------------|--------|-----------------------------------|
LSI |N.A. | 0.032000 MHz | yes | Mandatory for IWDG, DAC
IWDG1 |LSI | 0.032000 MHz | yes |
IWDG2 |LSI | 0.032000 MHz | yes |
LSE |N.A. | 0.032768 MHz | yes | Mandatory for DTS
RTC |LSE | 0.032768 MHz | yes |
TAMP |LSE | 0.032768 MHz | yes |
DTS |LSE | 0.032768 MHz | yes |
HSI |N.A. | 64.000000 MHz | yes |
I2C1 |HSI | 64.000000 MHz | yes | Rpi and peripherals
I2C2 |HSI | 64.000000 MHz | no |
I2C4 |HSI | 64.000000 MHz | yes | PMIC
I2C5 |HSI | 64.000000 MHz | yes | Rpi and peripherals
USART1 |HSI | 64.000000 MHz | no | Rpi (not used by default)
USART2 |HSI | 64.000000 MHz | yes | Bluetooth
USART3 |HSI | 64.000000 MHz | no |
UART4 |HSI | 64.000000 MHz | yes | Linux console
UART5 |HSI | 64.000000 MHz | no |
USART6 |HSI | 64.000000 MHz | no |
UART7 |HSI | 64.000000 MHz | yes | Arduino
UART8 |HSI | 64.000000 MHz | no | Rpi (not used by default)
HSE |N.A. | 24.000000 MHz | yes |
RTCDIV |HSE | 1.000000 MHz | yes | Only used when RTC source is HSE
USBPHYC |HSE | 24.000000 MHz | yes | USB PHY Ctrl for USB Host and OTG
USBPLL |USBPHYC | 48.000000 MHz | yes |
USBO |USBPLL | 48.000000 MHz | yes | USB OTG
USBH |USBPLL | 48.000000 MHz | yes | USB Host
STGEN |HSE | 24.000000 MHz | yes |
FDCAN2 |HSE | 24.000000 MHz | no | Rpi (not used by default)
ck_per |HSE | 24.000000 MHz | yes |
ADC2 |ck_per | 24.000000 MHz | yes | Analog watchdog
PLL1 |HSE | xxx MHz | yes |
PLL1P |PLL1 | xxx MHz | yes |
MPUDIV |PLL1P | xxx MHz | yes |
Cortex-A7 |PLL1P | xxx MHz | yes | 650 MHz or 1 GHz, see the information box above
PLL2 |HSE | 533.000000 MHz | yes |
PLL2P |PLL2 | 266.500000 MHz | yes |
AXI |PLL2P | 266.500000 MHz | yes | 266.5 MHz
FMC |AXI | 266.500000 MHz | no |
QSPI |AXI | 266.500000 MHz | no |
SAES |AXI | 266.500000 MHz | yes |
TZC |AXI | 266.500000 MHz | yes |
SYSRAM |AXI | 266.500000 MHz | yes |
ROM |AXI | 266.500000 MHz | yes |
AHB5 |AXI | 266.500000 MHz | yes | < 266MHz
CRYP1 |AHB5 | 266.500000 MHz | yes |
HASH1 |AHB5 | 266.500000 MHz | yes |
BKPSRAM |AHB5 | 266.500000 MHz | yes |
PKA |AHB5 | 266.500000 MHz | yes |
AHB6 |AXI | 266.500000 MHz | yes | < 266MHz
CRC1 |AHB6 | 266.500000 MHz | yes |
MDMA |AHB6 | 266.500000 MHz | yes |
MCE |AHB6 | 266.500000 MHz | yes |
APB4 |AXI | 133.250000 MHz | yes | < 133MHz
GPIOA-I |APB4 | 133.250000 MHz | yes |
APB5 |AXI | 66.625000 MHz | yes | < 133MHz
BSEC |APB5 | 66.625000 MHz | yes | < 67MHz
ETZPC |APB5 | 66.625000 MHz | yes |
DBGAPB |AXI | 133.250000 MHz | yes | JTAG & Coresight
DBGMCU |DBGAPB | 66.625000 MHz | yes |
PLL2Q |PLL2 | 266.500000 MHz | yes |
DCMIPP |PLL2Q | 266.500000 MHz | yes | < 533MHz
PLL2R |PLL2 | 533.000000 MHz | yes |
DDRPHYC |PLL2R | 533.000000 MHz | yes |
DDRC |PLL2R | 533.000000 MHz | yes |
DDRPERFM |PLL2R | 533.000000 MHz | yes |
PLL3 |HSE | 417.755859 MHz | yes |
PLL3P |PLL3 | 208.877930 MHz | yes |
MLAHB |PLL3P | 208.877930 MHz | yes | < 209MHz
SRAM1 |MLAHB | 208.877930 MHz | yes |
SRAM2 |MLAHB | 208.877930 MHz | yes |
SRAM3 |MLAHB | 208.877930 MHz | yes |
DFSDM |MLAHB | 208.877930 MHz | no | Rpi (not used by default)
AHB1 |MLAHB | 104.438965 MHz | yes | < 104.5MHz
AHB2 |MLAHB | 208.877930 MHz | yes | < 209MHz
DMA1 |AHB2 | 208.877930 MHz | yes |
DMA2 |AHB2 | 208.877930 MHz | yes |
DMA3 |AHB2 | 208.877930 MHz | yes |
DMAMUX1 |AHB2 | 208.877930 MHz | yes |
DMAMUX2 |AHB2 | 208.877930 MHz | yes |
APB1 |MLAHB | 104.438965 MHz | yes |
LPTIM1 |APB1 | 104.438965 MHz | no |
TIMG1 |MLAHB | 208.877930 MHz | yes |
TIM2 |TIMG1 | 208.877930 MHz | no | TIM Group 1
TIM3 |TIMG1 | 208.877930 MHz | no | TIM Group 1 - Rpi (not used by default)
TIM4 |TIMG1 | 208.877930 MHz | no | TIM Group 1 - Rpi (not used by default)
TIM5 |TIMG1 | 208.877930 MHz | no | TIM Group 1
TIM6 |TIMG1 | 208.877930 MHz | no | TIM Group 1
TIM7 |TIMG1 | 208.877930 MHz | no | TIM Group 1
TIMG2 |MLAHB | 208.877930 MHz | yes |
TIM1 |TIMG2 | 208.877930 MHz | no | TIM Group 2
TIM8 |TIMG2 | 208.877930 MHz | no | TIM Group 2 - Rpi (not used by default)
TIMG3 |MLAHB | 208.877930 MHz | yes |
TIM12 |TIMG3 | 208.877930 MHz | no | TIM Group 3
TIM13 |TIMG3 | 208.877930 MHz | no | TIM Group 3
TIM14 |TIMG3 | 208.877930 MHz | no | TIM Group 3 - Rpi (not used by default)
TIM15 |TIMG3 | 208.877930 MHz | no | TIM Group 3
TIM16 |TIMG3 | 208.877930 MHz | no | TIM Group 3
TIM17 |TIMG3 | 208.877930 MHz | no | TIM Group 3
APB3 |MLAHB | 104.438965 MHz | yes |
LPTIM2 |APB3 | 104.438965 MHz | no |
LPTIM3 |APB3 | 104.438965 MHz | yes |
LPTIM4 |APB3 | 104.438965 MHz | no |
LPTIM5 |APB3 | 104.438965 MHz | no |
SYSCFG |APB3 | 104.438965 MHz | yes |
VREFBUF |APB3 | 104.438965 MHz | yes |
HDP |APB3 | 104.438965 MHz | no |
APB6 |MLAHB | 104.438965 MHz | yes |
I2C3 |APB6 | 104.438965 MHz | no |
SPI5 |APB6 | 104.438965 MHz | no | Rpi (not used by default)
AHB4 |MLAHB | 208.877930 MHz | yes | < 209MHz
PWR |AHB4 | 208.877930 MHz | yes |
RCC |AHB4 | 208.877930 MHz | yes |
EXTI |AHB4 | 208.877930 MHz | yes |
PLL3Q |PLL3 | 24.573874 MHz | yes |
PLL3R |PLL3 | 11.290699 MHz | yes |
PLL4 |HSE | 600.000000 MHz | yes |
PLL4P |PLL4 | 50.000000 MHz | yes |
ETH1 |PLL4P | 50.000000 MHz | yes | 50MHz from the SoC
ETH2 |PLL4P | 50.000000 MHz | yes | 50MHz from the SoC
SDMMC1 |PLL4P | 50.000000 MHz | yes | µSD card
SDMMC2 |PLL4P | 50.000000 MHz | yes | Wifi
SPI1 |PLL4P | 50.000000 MHz | no |
SPI2 |PLL4P | 50.000000 MHz | no |
SPI3 |PLL4P | 50.000000 MHz | no |
SPDIFRX |PLL4P | 50.000000 MHz | no |
PLL4Q |PLL4 | 10.000000 MHz | yes |
LTDC |PLL4Q | 10.000000 MHz | yes | LTDC display pixel clock
SAI1 |PLL4Q | 10.000000 MHz | no | Rpi (not used by default)
SAI2 |PLL4Q | 10.000000 MHz | no |
SPI4 |PLL4Q | 10.000000 MHz | no |
PLL4R |PLL4 | 50.000000 MHz | yes |
ADC1 |PLL4R | 50.000000 MHz | no |
RNG |PLL4R | 50.000000 MHz | yes |
CSI |N.A. | 4.000000 MHz | yes | Mandatory for IO compensation
-----------------------|---------|-----------------|--------|-----------------------------------|
3.1.2. Device tree
As mentioned in previous chapters, RCC configuration is done in two steps, first by the first stage boot loader (FSBL TF-A) and then by the secure OS (OP-TEE).
Here are the corresponding device tree rcc sub node properties in fdts/stm32mp135f-dk.dts consumed by the first stage boot loader (FSBL TF-A) to configure the clock tree required to boot on STM32MP135F-DK Discovery kit :
&rcc {
st,clksrc = <
CLK_MPU_PLL1P
CLK_AXI_PLL2P
CLK_MLAHBS_PLL3
CLK_CKPER_HSE
CLK_RTC_LSE
CLK_SDMMC1_PLL4P
CLK_SDMMC2_PLL4P
CLK_STGEN_HSE
CLK_USBPHY_HSE
CLK_I2C4_HSI
CLK_USBO_USBPHY
CLK_I2C12_HSI
CLK_UART2_HSI
CLK_UART4_HSI
CLK_SAES_AXI
>;
st,clkdiv = <
DIV(DIV_AXI, 0)
DIV(DIV_MLAHB, 0)
DIV(DIV_APB1, 1)
DIV(DIV_APB2, 1)
DIV(DIV_APB3, 1)
DIV(DIV_APB4, 1)
DIV(DIV_APB5, 2)
DIV(DIV_APB6, 1)
DIV(DIV_RTC, 0)
>;
st,pll_vco {
pll2_vco_1066Mhz: pll2-vco-1066Mhz {
src = <CLK_PLL12_HSE>;
divmn = <2 65>;
frac = <0x1400>;
};
pll3_vco_417Mhz: pll3-vco-417Mhz {
src = <CLK_PLL3_HSE>;
divmn = <1 33>;
frac = <0x1a04>;
};
pll4_vco_600Mhz: pll4-vco-600Mhz {
src = <CLK_PLL4_HSE>;
divmn = <1 49>;
};
};
/* VCO = 1066.0 MHz => P = 266 (AXI), Q = 266, R = 533 (DDR) */
pll2:st,pll@1 {
compatible = "st,stm32mp1-pll";
reg = <1>;
st,pll = <&pll2_cfg1>;
pll2_cfg1: pll2_cfg1 {
st,pll_vco = <&pll2_vco_1066Mhz>;
st,pll_div_pqr = <1 1 0>;
};
};
/* VCO = 417.8 MHz => P = 209, Q = 24, R = 209 */
pll3:st,pll@2 {
compatible = "st,stm32mp1-pll";
reg = <2>;
st,pll = <&pll3_cfg1>;
pll3_cfg1: pll3_cfg1 {
st,pll_vco = <&pll3_vco_417Mhz>;
st,pll_div_pqr = <1 16 1>;
};
};
/* VCO = 600.0 MHz => P = 50, Q = 10, R = 100 */
pll4:st,pll@3 {
compatible = "st,stm32mp1-pll";
reg = <3>;
st,pll = <&pll4_cfg1>;
pll4_cfg1: pll4_cfg1 {
st,pll_vco = <&pll4_vco_600Mhz>;
st,pll_div_pqr = <11 59 5>;
};
};
};
Here are the corresponding device tree rcc sub node properties in core/arch/arm/dts/stm32mp135f-dk.dts consumed by the secure OS (OP-TEE) to configure the clock tree above:
&rcc {
st,clksrc = <
CLK_MPU_PLL1P
CLK_AXI_PLL2P
CLK_MLAHBS_PLL3
CLK_RTC_LSE
CLK_MCO1_HSE
CLK_MCO2_DISABLED
CLK_CKPER_HSE
CLK_ETH1_PLL4P
CLK_ETH2_PLL4P
CLK_SDMMC1_PLL4P
CLK_SDMMC2_PLL4P
CLK_STGEN_HSE
CLK_USBPHY_HSE
CLK_I2C4_HSI
CLK_I2C5_HSI
CLK_USBO_USBPHY
CLK_ADC2_CKPER
CLK_I2C12_HSI
CLK_UART1_HSI
CLK_UART2_HSI
CLK_UART35_HSI
CLK_UART4_HSI
CLK_UART6_HSI
CLK_UART78_HSI
CLK_SAES_AXI
CLK_DCMIPP_PLL2Q
CLK_LPTIM3_PCLK3
CLK_RNG1_PLL4R
>;
st,clkdiv = <
DIV(DIV_MPU, 1)
DIV(DIV_AXI, 0)
DIV(DIV_MLAHB, 0)
DIV(DIV_APB1, 1)
DIV(DIV_APB2, 1)
DIV(DIV_APB3, 1)
DIV(DIV_APB4, 1)
DIV(DIV_APB5, 2)
DIV(DIV_APB6, 1)
DIV(DIV_RTC, 0)
DIV(DIV_MCO1, 0)
DIV(DIV_MCO2, 0)
>;
st,pll_vco {
pll1_vco_2000Mhz: pll1-vco-2000Mhz {
src = <CLK_PLL12_HSE>;
divmn = <1 82>;
frac = <0xAAA>;
};
pll1_vco_1300Mhz: pll1-vco-1300Mhz {
src = <CLK_PLL12_HSE>;
divmn = <2 80>;
frac = <0x800>;
};
pll2_vco_1066Mhz: pll2-vco-1066Mhz {
src = <CLK_PLL12_HSE>;
divmn = <2 65>;
frac = <0x1400>;
};
pll3_vco_417Mhz: pll3-vco-417Mhz {
src = <CLK_PLL3_HSE>;
divmn = <1 33>;
frac = <0x1a04>;
};
pll4_vco_600Mhz: pll4-vco-600Mhz {
src = <CLK_PLL4_HSE>;
divmn = <1 49>;
};
};
/* VCO = 1300.0 MHz => P = 650 (CPU) */
pll1: st,pll@0 {
compatible = "st,stm32mp1-pll";
reg = <0>;
st,pll = <&pll1_cfg1>;
pll1_cfg1: pll1_cfg1 {
st,pll_vco = <&pll1_vco_1300Mhz>;
st,pll_div_pqr = <0 1 1>;
};
pll1_cfg2: pll1_cfg2 {
st,pll_vco = <&pll1_vco_2000Mhz>;
st,pll_div_pqr = <0 1 1>;
};
};
/* VCO = 1066.0 MHz => P = 266 (AXI), Q = 266, R = 533 (DDR) */
pll2: st,pll@1 {
compatible = "st,stm32mp1-pll";
reg = <1>;
st,pll = <&pll2_cfg1>;
pll2_cfg1: pll2_cfg1 {
st,pll_vco = <&pll2_vco_1066Mhz>;
st,pll_div_pqr = <1 1 0>;
};
};
/* VCO = 417.8 MHz => P = 209, Q = 24, R = 11 */
pll3: st,pll@2 {
compatible = "st,stm32mp1-pll";
reg = <2>;
st,pll = <&pll3_cfg1>;
pll3_cfg1: pll3_cfg1 {
st,pll_vco = <&pll3_vco_417Mhz>;
st,pll_div_pqr = <1 16 36>;
};
};
/* VCO = 600.0 MHz => P = 50, Q = 10, R = 50 */
pll4: st,pll@3 {
compatible = "st,stm32mp1-pll";
reg = <3>;
st,pll = <&pll4_cfg1>;
pll4_cfg1: pll4_cfg1 {
st,pll_vco = <&pll4_vco_600Mhz>;
st,pll_div_pqr = <11 59 11>;
};
};
st,clk_opp {
/* CK_MPU clock config for MP13 */
st,ck_mpu {
cfg_1 {
hz = <650000000>;
st,clksrc = <CLK_MPU_PLL1P>;
st,pll = <&pll1_cfg1>;
};
cfg_2 {
hz = <1000000000>;
st,clksrc = <CLK_MPU_PLL1P>;
st,pll = <&pll1_cfg2>;
};
};
};
};